Both were part of the Louisiana Purchase. Named after the Arkansas and Kansas river, which were named for the plural of the Arkansa and Kansa Indians.

My take: Arkansas, settled by slower migration of southerners, and closer to the metropolis of New Orleans, kept the French pronunciation already in use. Kansas, settled later (and probably more quickly) by easterners not familiar with the original pronunciation of the French name on the map, lost that French pronunciation, and applied an English pronunciation to the French spelling.

Arkansas used to be part of Kansas, but then they wanted to name it "Arkanoid" in honor of the infamous video game. To compromise, they came up with "Arkansas" (which was, incidently, pronounced "ARE-KANN-ZISS").

Later, they wanted to change the name of the state again (they're wishy-washy people, they are), this time to "Coleslaw". Well, another compromise was reached, and the pronunciation of the state was changed to the now-current "ARE-KENN-SAW".

The Arkansas River which runs from Colorado, though Kansas, Oklahoma and Arkansas, is pronouced like the state of Arkansas in Colorado, Oklahoma, and Arkansas, but while in Kansas, it is pronounced "Are Kansas".

If you ask something about the Arkansas River in Kansas and pronouce it like the former home state of Bill Clinton, you'll get laughed at. I kid you not.